From: Fred Bloggs
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Subject: Re: circuit wanted-SQUARE PULSE GENERATOR
References: <1038144646.265208@athprx02> <email@example.com>
Date: Mon, 25 Nov 2002 15:59:15 GMT
NNTP-Posting-Date: Mon, 25 Nov 2002 07:59:15 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Winfield Hill wrote:
> GEORGE BASILIOU wrote...
>>Unfortunately i'm not good in designing Circuits. Has anyone to
>>suggest me any circuit/page to find a square pulse generator,
>>variable from 1 to 1000 Hz, with 1 Hz step?
>>I prefer not using 555. I want the circuit to be accurate.
> The poor 555 gets so much abuse. What a great little chip, two
> precision comparators, a flip-flop, a large discharge transistor,
> serious output driving capability, a trimmable voltage divider.
> These pieces are an excellent starting point for simple accurate
> oscillators like George wants. If we choose a CMOS 555 version,
> like Intersil's ICM7555, we can use low charging currents, like
> 0.33uA, and get accurate operation even down to 1Hz and below
> with a 0.1uF film capacitor.
> While Jesper Hansen's design for the AT90S2313 is appealing, see
> if the simplicity of a few analog parts can't also be appealing.
> Here's my design that uses a common 10k-ohm 10-turn pot with a
> turns-counting dial for precision frequencies from 0 to 1000Hz.
> The IC1 Q1 circuit creates a charging current I = Vr R2 / (R1 R3),
> used by the 555 to make a frequency f = I / C Vth = I Vr / 2/3 C
> Note, we can adjust the 2/3 term to eliminate the capacitor's 5%
> tolerance error, the resistor values, and Q1's beta error. The
> 7555's DISCHARGE pin current limits at 40mA and then 20 ohms of
> internal resistance below 1V, so R4 and C2 provide a fixed 25us
> time for thoroughly discharging C each cycle.
> After calibration the frequency is proportional to the setting of
> the 10-turn pot with its counting-dial indicator from 0 to 1 kHz.
> 10k 10T pot
> ,--- R2 --+-------, +5 -- 1.65k -,
> | __ | | |
> ,---+--|- \ | R1 1k calibrate
> | | >--' 15.0k pot <------,
> R3 ,--|+_/ | | |
> 10.0k | ,----------+ 4.02k |
> | | | __ | | 2/3 Vr
> gnd | '-|- \ |/V Q1 +5 gnd +/-5%
> | | >--| 2n3906 ___|___________ |
> +--------|+_/ |\ | | |
> | IC1 | | CONTROL |--' Vth
> Vr +5 LT1013 | 6 | |
> +-----------| THRESHOLD |
> | 2 | OUT |-----
> 0 to 1kHz precision +-- R4 -+---| TRIGGER |
> 555-type oscillator |\ | | |
> | \____ |_7_| DISCHARGE |
> 3 R2 C _|_ | |_______________|
> f = --------- ___ C2_|_ |
> 2 C R1 R3 film | ___ gnd IC2
> 0.1uF | 0.001 | ICL7555
> | |
> gnd gnd
> IC1 is powered from +15V, and must be a low-offset-voltage opamp
> because at 1Hz the current-programming voltage is only 5mV.
> There's lots more to say about this circuit, but it's that time of
> the year again; I have to go finish burying the fig tree.
Hmmm- those turns-counting 10T are a bit on the pricey side. Instead of
the total-digital vs total-analog approach, maybe the new DAC-pots hold
promise for an accessible and economical hybrid solution, a circuit that
does not require a PC to operate and low cost all the way. Some of the
DAC pots have very simple one-button interfaces, some roll over -some do
not, some auto-store on power down- some reset to mid-range on power up
etc, some are 8-bit and some are >10bit, the flexibility seems infinite.